Field of the Invention
The invention relates to a memory cell configuration including at least one magnetoresistive element, and a PRODUCTION METHOD.
Within the specialty, the term magnetoresistive element refers to a structure that includes at least two ferromagnetic layers and one intermediate non-magnetic layer. GMR elements, TMR elements, and CMR elements are distinguished according to their layer structures (see S. Mengel, xe2x80x9cTechnologieanalyse Magnetismusxe2x80x9d, Vol. 2, XMR Technologies, VDI Technologiezentrum Physikalische Technologien (August 1997)).
The term GMR element is used for layer structures that include at least two ferromagnetic layers and one intermediate nonmagnetic conductive layer and that exhibit what is known as GMR (Giant Magnetoresistance). GMR effect refers to the fact that the electrical resistance of the GMR element is dependent upon whether the magnetizations in the two ferromagnetic layers are oriented parallel or antiparallel relative to each other. GMR effect is large compared to what is known as AMR effect (Anisotropic Magnetoresistance). AMR effect refers to the fact that the resistance in magnetized conductors is different parallel to the magnetization direction and perpendicular to it. AMR effect is a matter of a volume effect that emerges in the ferromagnetic monolayers.
The term TMR element is used for tunneling magnetoresistance layer structures that include at least two ferromagnetic layers and one intermediate insulating, non-magnetic layer. The insulating layer is so thin that a tunnel current is induced between the two ferromagnetic layers. These layer structures likewise exhibit a magnetoresistive effect, which is effectuated by a spin-polarized tunnel current through the insulating non-magnetic layer that is disposed between the two ferromagnetic layers. In this case, also, the electrical resistance of the TMR element depends upon whether the magnetizations in the two ferromagnetic layers are oriented parallel or antiparallel to each other. The relative change in resistance equals from 6% to approx. 40% at room temperature.
Another magnetoresistance effect, which is called CMR (Colossal Magnetoresistance) due to it size (relative change in resistance of from one-hundred to four-hundred percent (100-400%) at room temperature), requires a high magnetic field for switching between the magnetization states owing to its high coercive forces.
The utilization of GMR elements as storage elements in a memory cell configuration has been suggested (see e.g. D. D.
Tang et al, IEDM 95, pp. 997-99; J. M. Daughton, Thin Solid Films, v. 216 (1992): 162-68; Z. Wang et al, Journal of Magnetism and Magnetic Materials, v. 155 (1996): 161-63). The storage elements are connected in series by way of read lines. Extending perpendicular to these are word lines, which are insulated against both the read lines and the storage elements. Due to the current flowing in each word line, signals that are applied to the word lines bring about a magnetic field, which, if it is strong enough, influences the underlying storage elements. For writing information, x/y lines are used, which cross at the memory cell that is being written. They are charged with signals that produce a sufficient magnetic field at the crossing to cause remagnetization. In this process, the magnetization direction in one of the two ferromagnetic layers is switched. By contrast, the magnetization direction in the other of the two ferromagnetic layers remains unchanged. The magnetization direction in the latter ferromagnetic layer is maintained with the aid of a neighboring antiferromagnetic layer, which keeps the magnetization direction fixed, or by increasing the switching threshold for this ferromagnetic layer using a different material or a different dimensioning, for instance layer thickness, compared to the former ferromagnetic layer.
U.S. Pat. No. 5,541,868 to Prinz and U.S. Pat. No. 5,477,482 to Prinz propose annular storage elements based on GMR effect. A storage element includes a stack having at least two annular ferromagnetic layer elements and one intermediately disposed non-magnetic conductive layer element connected between two lines. The ferromagnetic layer elements have different material compositions. One of the ferromagnetic layer elements is magnetically hard, while the other is magnetically softer. To write the information, the magnetization direction in the magnetically softer layer element is switched, while the magnetization direction in the magnetically harder layer element is maintained.
Another memory cell configuration containing annular storage elements based on GMR effect is taught in International Publication WO 96/25740. These include layer elements composed of two magnetic materials, one of which has a high coercive strength, and the other of which has a low coercive strength. To actuate the magnetoresistive element, two driver lines are provided, both of which run through the middle of the annular GMR element. The switching of the magnetization direction is accomplished with the aid of a magnetic field that is induced by currents in the two driver lines.
For purposes of switching the magnetization direction, a current flows between the two lines between which the GMR element is connected and also via the storage element. The magnetic field that is induced by this current is used for modifying the magnetization direction.
Because the two driver lines run through the middle of the annular GMR element and must be insulated against each other, the packing density that can be achieved with this configuration is limited.
It is accordingly an object of the invention to provide a memory cell configuration and PRODUCTION METHOD that overcomes the hereinafore-mentioned disadvantages of the heretofore-known devices of this general type and that provides a memory cell configuration having at least one magnetoresistive element. Such a configuration is insensitive to external magnetic interference fields, functional for magnetoresistive element s with both TMR and GMR effects, and producible with higher packing densities than the prior art. Furthermore, a method is laid out for producing such a memory cell configuration.
With the foregoing and other objects in view, there is provided, in accordance with the invention, a memory cell configuration. The memory cell configuration includes a first line, a second line, and a magnetoresistive element. The second line is crossed by the first line to define a crossing region. The magnetoresistive element has an annular cross-section in a layer plane and layer elements stacked perpendicular to the layer plane. The magnetoresistive element is disposed in the crossing region. The first line and the second line are disposed in the crossing region on opposing sides of the magnetoresistive element relative to the layer plane. At least one of the first line and the second line including at least one first portion has a predominant current component oriented parallel to the layer plane and one second portion having a predominant current component oriented perpendicular to the layer plane, in the overlap region.
With the objects of the invention in view, there is also provided a method for producing a memory cell configuration including the following steps. The first step is creating a first line on a main surface of a substrate. The next step is forming a magnetoresistive element exhibiting an annular cross-section in a layer plane by depositing and structuring a first ferromagnetic layer, a non-magnetic layer, and a second ferromagnetic layer. The next step is disposing the magnetoresistive element in the crossing region by creating a second line crossing the first line. The next step is creating at least one of said first line and said second line in the overlap region. The next step is including a first portion of said at least one of said first line and said second line in the overlap region with a predominant current component oriented parallel to the layer plane, and a second portion with a predominant current component oriented perpendicular to the layer plane.
The memory cell configuration includes at least one magnetoresistive element, whose cross-section in a layer plane is annular. The magnetoresistive element includes layer elements that are stacked perpendicular to the layer plane. The utilization of a magnetoresistive element with an annular cross-section guarantees a higher insensitivity to external magnetic interference fields, because external magnetic interference fields are rather homogenous over the extent of the annular element and thus substantially without effect. Given the use of xcexc-metal, additional shielding measures can be forgone.
Since there is a closed magnetic flux in an annular ferromagnetic layer element, magnetic leakage fields escape to the outside environment most frequently during the remagnetization process. Therefore, layer elements of a magnetoresistive element or adjoining elements are almost entirely magnetically decoupled. Thus, a plurality of identical magnetoresistive elements can be provided in the memory cell configuration in a high packing density.
Annular layer elements exhibit two stable magnetization states; i.e., the magnetization flux is closed either in the clockwise or counterclockwise direction. Both states are very stable, and the transitions from one into the other are insensitive to defects and geometric irregularities. The probability of information losses due to irreversible magnetization processes is thus lower than in conventional, singly integrated element structures.
The memory cell configuration also includes a first line and a second line, which cross. In the crossing region between the first and second lines, the magnetoresistive element is disposed. The first and second lines are thus disposed at the crossing on different sides of the magnetoresistive element relative to the layer plane. The first and/or second lines include at least one first and one second line portion. The first line portion is oriented such that the predominant current component therein is aligned parallel to the layer plane, whereas the predominant current component in the second line portion in the crossing region between the first and second lines is aligned perpendicular to the layer plane. Specifically, the first portion runs parallel to the layer plane, and the second line portion crosses a plane that is parallel to the layer plane in the crossing region between the first and second lines. In particular, the first line and/or the second line are bent perpendicular to the layer plane.
At the location of the annular magnetoresistive elements, the currents flowing through the so-constructed lines generate a magnetic field that is suitable for remagnetizing the magnetoresistive elements in the write operation. Both the azimuthal (circular) magnetic fields of the vertical current components in the layer plane and the lateral magnetic field components of the parallel current components, i.e. the magnetic field components, which are oriented in the layer plane perpendicular to the longitudinal direction of the line, contribute to the remagnetization field. The current components that are parallel to the layer plane contribute to remagnetization because the first line portions of the first and second lines are different distances from the annular magnetoresistive element and thus do not compensate each other.
By virtue of these types of lines, it is possible to realize memory cell configurations that can be produced more easily and with larger packing densities than hitherto. The first and second lines, which cross at the location of the storage element, are sufficient for writing and reading. Additional lines, for instance through the annular storage elements, are not necessary, in contrast to the solution known from WO 96/25740. The result is a lower surface consumption per memory cell.
Furthermore, the memory cell configuration can be realized either with a magnetoresistive element based on GMR effect or a magnetoresistive element based on TMR effect, since, unlike the solution known from U.S. Pat. Nos. 5,477,482 and 5,541,868, no current is needed across the magnetoresistive element for generating the magnetic switching field.
Both the first and second lines expediently include at least one first portion in which the predominant current component is oriented parallel to the layer plane and one second portion in which the predominant current component is oriented perpendicular to the layer plane. If the first and second lines are wired up in such a way that the current through the second portion of the first line and the current through the second portion of the second line flow in the same direction, the azimuthal magnetic fields of these currents constructionally overlap and reinforce one another at the location of the magnetoresistive element. This makes selective writing in memory cell fields possible.
If the magnetoresistive element is interposed between the first and second lines, the stored information can be read over the first and second lines. To accomplish this, the resistance of the magnetoresistive element is evaluated. This can be accomplished by measuring the absolute resistance of the magnetoresistive element, measuring the change in resistance in the switching of the magnetoresistive element, or comparing the resistance to a neighboring magnetoresistive element""s known magnetization state. Any method of evaluating the resistance of the magnetoresistive element is suitable for reading the stored information.
The magnetoresistive element expediently includes a first ferromagnetic layer element, a non-magnetic layer element, and a second ferromagnetic layer element, respectively, whereby the non-magnetic layer element is disposed between the first and second ferromagnetic layer elements. The magnetoresistive element can be based on either GMR or TMR. The utilization of a TMR magnetoresistive element is preferable, owing to the relatively larger resistance, the consequent reduction in power consumption, and the usually larger magnetoresistance effect. Beyond this, the magnetoresistive element can be based on a CMR effect, provided that the configuration is able to generate the required magnetic switching fields.
The first ferromagnetic layer element and the second ferromagnetic layer element expediently contain at least one of the following elements: Fe, Ni, Co, Cr, Mn, Gd, Dy, and Bi. The first ferromagnetic layer element and the second ferromagnetic layer element differ with respect to magnetic hardness and/or layer thickness.
The first and second ferromagnetic layer elements expediently have a thickness of between 2 nm and 20 nm perpendicular to the layer plane. In the case of a TMR effect, the nonmagnetic layer element expediently contains Al2O3, NiO, HfO2, TiO2, NbO, or SiO2, individually or in combination, and forms a thickness of between 1 and 4 nm perpendicular to the layer plane. In the case of a GMR element, the non-magnetic layer element expediently contains Cu, Au, Ag or Al, individually or in combination, and forms a thickness of between 2 and 5 nm perpendicular to the layer plane. The first ferromagnetic layer element, the second ferromagnetic layer element, and the non-magnetic layer element have dimensions between 50 and 400 nm parallel to the layer plane.
For storing large quantities of data, the memory cell configuration includes a number of identical magnetoresistive elements disposed in a matrix. In addition, a number of identical first lines and identical second lines are provided. The first and second lines cross. Each magnetoresistive element is disposed in the crossing region between one of the first lines and one of the second lines. The first and/or second lines each include first portions, in which the predominant current component is oriented parallel to the layer plane, and second portions, in which the predominant current component is oriented perpendicular to the layer plane, in alternation. Because the annular magnetoresistive elements are nearly magnetically decoupled, a high packing density can be achieved.
Preferably, both the first and second lines include first and second portions, so that a selective writing into the individual memory cells is possible.
In accordance with a development of the invention, the first and second portions of one of the first lines and/or one of the second lines are disposed in such a way that the appertaining line has a strip-shaped cross-section parallel to the layer plane. In this development, a surface consumption of 4 F2 per cell can be achieved, F being the minimum structural size that can be produced in the respective technology, to the extent that both the width of the lines parallel to the layer plane and the spacing between adjoining lines equal F. In this configuration, the overlap of the by the vertical current components in the first and/or second lines produces an azimuthal magnetic field in the layer plane at the location of each annular storage element. This field is primarily responsible for the remagnetization of the annular magnetoresistive elements. Magnetic field contributions, which stem from the current components that are parallel to the layer plane, lead to an asymmetry of the resulting magnetic switching field, which has a positive effect with respect to reducing switching field thresholds.
In a separate development of the memory cell configuration, the magnetoresistive elements are disposed in rows and columns between the first and second lines, and the layer plane spreads through the center planes of the magnetoresistive elements. The direction of the rows and the direction of the columns run parallel to the layer plane, whereby the direction of the rows crosses the direction of the columns. The projections of the first portions of one of the first lines on the layer plane are respectively disposed between adjoining magnetoresistive elements of this row in such a way that the projections are laterally staggered relative to the connecting lines through the magnetoresistive elements of these cells. The projection of the first portions of one of the second lines on the layer plane is disposed between adjoining magnetoresistive elements of one of the columns, whereby the projection is laterally offset relative to a connecting line between the adjoining magnetoresistive elements. The projections of first portions, which are adjacently disposed along one of the lines, on the layer plane are set off to opposite sides relative to the respective connecting lines. The projections of the first and second lines on the layer plane are therefore wavy instead of being elongated rectangles. In this development, double-digit symmetrical local azimuthal magnetic fields are produced at the location of the magnetoresistive elements. The per-cell surface consumption equals 9 F2.
In this development of the memory cell configuration, magnetic switching fields of greater symmetry, namely double-digit symmetry, are generated in the layer plane at the location of the annular elements. This development expediently includes the following features:
The projection of the first and second lines on the layer plane are strips whose center lines and margins are parallel wavy polygonal lines.
The structures in the wavy strips repeat periodically, whereby the wavy strips oscillate about a center longitudinal direction.
The adjoining projection strips of the first and second lines are offset relative to one another by a half-period in the longitudinal direction.
The projection strips of the first lines cross those of the second lines at the xe2x80x9czeroesxe2x80x9d of the wave strips, whereby the center longitudinal directions form a right angle, though the strips extend parallel to one another section by section. The respective crossing of the projection strip with the respective center longitudinal direction is referred to as the zero.
The annular magnetoresistive elements are disposed in the layer plane in rows and columns at the crossings between the first and second lines.
The first and second lines are bent perpendicular to the layer plane at the crossings, so that second line portions having vertical current components exist at these locations.
Given the constructional overlap of the magnetic fields generated by the vertical current components of the first and second lines, and given sufficient current intensities, double-digit symmetrical switching fields can be generated at the location of the annular magnetoresistive elements in this configuration.
This development can be realized with a per-cell surface consumption of 9 F2. To this end, the memory cell configuration includes the following additional features:
The period of the wavy strips equals 6F; their amplitude equals F/2.
The strips have a minimum width perpendicular to their longitudinal direction, and a minimum spacing, of F.
The projection strips of the first and second lines extend parallel to one another in segments with a length of F.
The annular storage elements are disposed in rows and columns in the layer plane at the crossings between the first and second lines at intervals of 3F.
To produce the memory cell configuration, a first line is created on a main surface of a substrate. The magnetoresistive element, which has an annular cross-section in a layer plane, is formed by depositing and structuring a first ferromagnetic layer, a non-magnetic layer, and a second ferromagnetic layer. A second line is created, which crosses the first line in such a way that the magnetoresistive element is disposed in the crossing region. The first line and/or the second line are created such that they include at least one first portion, in which the predominant current component is oriented parallel to the layer plane, and a second portion, in which the predominant current component is oriented perpendicular to the layer plane.
The first ferromagnetic layer, the non-magnetic layer, and the second ferromagnetic layer are preferably structured with the aid of the same mask.
To structure the annular magnetoresistive element, a self-aligning process is preferably used. To accomplish this, an opening is created in a layer that is disposed at a main surface of a substrate, and a conformal layer is deposited over the edges of said opening. Upon the anisotropic etchback of the conformal layer, an annular spacer emerges at the edges, which is used as a mask for the anisotropic structuring. If the opening is created with a dimension of F, magnetoresistive elements can be produced with an outer diameter of F and an inner diameter of less than F.
The first and second lines are preferably produced in two steps. First the lower segments of the first or second line are produced, and then the upper segments of the first or second line are produced. The projection of the lower segments and the projection of the upper segments of the respective line on the main surface of the substrate partially overlap, so that contiguous and bent first and second lines emerge. The second portions, in which vertical current components relative to the layer plane occur, emerge in the overlap regions of the lower and upper segments of the respective line. Intervening parts of the lower segments and upper segments represent the first portions, which extend parallel to the layer plane.
In the production of the lower segments of the first and second lines, a first metallization plane, which is usually referred to by experts as metal 1, and a second metallization plane, which is usually referred to as metal 2, are simultaneously formed in the periphery of the memory cell configuration. In the production of the top segments of the first and second lines, first contacts, referred to as Via 1, and second contacts, referred to as Via 2, are simultaneously formed in the periphery.
The first lines of the cell field are expediently bonded by way of the first metallization plane of the periphery, and the second lines of the cell field are bonded by way of the second metallization plane.
The first and second lines are expediently produced using the Damascene technique. To this end, a first insulating layer is deposited and then structured with the aid of photolithography steps and anisotropic plasma etching steps (RIE) so as to be removed in the region of the subsequently created first metallization plane of the periphery and lower segments of the first lines of the cell field. A first conductive layer or a first conductive layer system is deposited and structured by a planarizing etching technique, for instance CMP. The lower segments of the first lines and the first metallization planes of the periphery are formed in this way. Next, a second insulating layer is deposited and structured with the aid of photolithography steps and anisotropic etching steps so as to be removed in the region of the subsequently produced first contacts of the periphery and upper segments of the first line. The first contacts and the upper segments of the first line are formed by depositing a second conductive layer or a second conductive layer system and structuring it by a planarization technique such as CMP.
In a corresponding manner, the lower segments of the second line and the second metallization plane of the periphery are formed by depositing and structuring a third insulating layer and a third conductive layer or layer system, and the upper segments of the second line and the second contacts of the periphery are formed by depositing and structuring a fourth insulating layer and a fourth conductive layer.
Because the first and second lines are each produced in two steps, the production of the memory cell configuration can be easily integrated into a multi-layer wiring process. The deposition and structuring steps that are required in order to produce the peripheral metallization planes and the contacts that are needed between them (also known as vias), are also used to form the lower and upper segments of the first and second lines. The formation of the lower and upper segments of the first lines of the cell field in the same procedure as the first metallization plane (metal 1) and the first contact plane (via 1) of the periphery. Similarly, the lower and upper segments of the second lines are formed simultaneously with the second metallization plane (metal 2) and the second contact plane (via 2).
This procedure also solves the technical problem that a much larger vertical spacing exists between mutually overlying metallization planes of the periphery than between the first and second lines of the cell field. The vertical spacing between the first and second lines in the cell field is determined by the dimensions of the magnetoresistive element, which typically equal 20 to 40 nm. The spacing between adjoining metallization planes of the periphery must be significantly larger in order to reduce parasitic capacitances. In a 0.35-xcexcm technology, it typically equals 350 to 400 nm. The inventive method solves this problem without additional metallization planes, additional topography, or vias with large aspect ratios.
Other features which are considered as characteristic for the invention are set forth in the appended claims.
Although the invention is illustrated and described herein as embodied in a memory cell configuration and PRODUCTION METHOD, it is nevertheless not intended to be limited to the details shown, since various modifications and structural changes may be made therein without departing from the spirit of the invention and within the scope and range of equivalents of the claims.
The construction and method of operation of the invention, however, together with additional objects and advantages thereof will be best understood from the following description of specific embodiments when read in connection with the accompanying drawings.